Combine all code coverages in CI (#1259)

**Pull Request Checklist**
- [ ] Fixes #<!--issue number goes here-->
- [ ] Tests added
- [ ] Documentation/examples added
- [ ] [Good commit messages](https://cbea.ms/git-commit/) and/or PR
title

**Description of PR**
Currently, the codecov reports definitely covered line in PR pages.
So in this pr, I tried to combine all code coverages in CI and report
them to codecov server.

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Signed-off-by: Ukjae Jeong <jeongukjae@gmail.com>

Makefile

@@ -48,7 +48,7 @@ test:  ## Run tests for Hera
 
 .PHONY: test-type-hints
 test-type-hints:  ## Run type hint tests for Hera
-	@poetry run python -m pytest -k "typehints"
+	@poetry run python -m pytest --cov-append -k "typehints"
 
 .PHONY: workflows-models
 workflows-models: ## Generate the Workflows models portion of Argo Workflows